FPS problem with CS 1.6

[SavAndjelkovic]

Hi.. I have problem with my FPS when I play CS 1.6 .
It is 100 fps, but it falls while I shoot. I don't know why. :S
I have :

Grafic Card : ATI Radeon HD 5400 Series [sync master]
Procesor : AMD Sempron(tm) 2800 +
Ram Memory : 1.25 GB of Ram
Physical Address Extension, Service Pack 2 .

I would like to get my fps does not move from 100. Please help me.
Thanks.